The invention relates to a method and apparatus for processing video pictures, especially for dynamic false contour effect compensation. This method and apparatus is usable in display devices, such as matrix displays like plasma display panels (PDP), display devices with digital micro mirror arrays (DMD) and all kinds of displays based on the principle of duty cycle modulation (pulse width modulation) of light generation.
The plasma display technology now makes it possible to achieve flat colour panels of large size and with limited depth without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever allowed.
A plasma display panel utilizes a matrix array of discharge cells, which could only be “on” or “off”. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the emission, a PDP controls the grey levels by modulating the number of light pulses per frame (sustain pulses). The eye will integrate this time-modulation over a period corresponding to the eye-time response.
Since the video amplitude determines the number of light pulses occurring at a given frequency, more amplitude means more light pulses and thus more “on time”. For this reason this kind of modulation is known as PWM, Pulse Width Modulation. To establish a concept for this PWM, each frame will be decomposed in sub-periods called “sub-fields”. For producing the small light pulses, an electrical discharge appears in a gas filled cell, called plasma cell and the produced UV radiation will excite a coloured phosphor, which emits light. In order to select which cell should be lighted, a first selected operation called “addressing” creates a charge in the cell to be lighted. Each plasma cell can be considered as a capacitor, which keeps the charge for a relative long time. Afterwards, a general operation called “sustaining” applied during the lighting period will accelerate the charges in the cell, produce further charges and excite some of the charges in the cell. Only in the cells addressed during the first selected operation, this excitation of charges takes place and UV radiation is generated when the excited charges go back to their neutral state. The UV radiation excites a phosphorous for light emission. The discharge of the cell is made in a very short period and some of the charges in the cell remain. With the next sustain pulse, the charge is utilized again for the generation of UV radiation and the next light pulse will be produced. During the whole sustain period of each specific sub-field, the cell will be lighted in small pulses. At the end an erase operation will remove all the charges to prepare a new cycle.
On one hand, the plasma display technology gives the possibility of nearly unlimited screen size, also of attractive thickness, but on the other hand, it generates new kinds of artefacts, which could damage the picture quality. Most of these artefacts are different from the known artefacts occurring on classical CRT colour picture tubes. It is mainly this different appearance of the artefacts that make them more visible to the viewer, since the viewer is used to see the well-known TV artefacts.
The invention mainly deals with a new specific artefact, which is called “dynamic false contour effect” since it corresponds to disturbances of grey levels and colours in form of an apparition of coloured edges in the picture when an observation point on the matrix screen moves. This kind of artefact is enhanced when the image has a smooth gradation, like when the skin of a person is being displayed (e.g. displaying of a face or of an arm, etc.). In addition, the same problem occurs on static images when observers are shaking their heads and that leads to the conclusion that such a failure depends on the human visual perception and happens on the retina of the eye.
In the prior art some approaches are already known to compensate for the false contour effect. As a false contour effect is directly related to the sub-field organization of the used plasma technology, one approach is to make an optimisation of the sub-field organization of the plasma display panels. The sub-field organization will be explained in greater detail below, but for the moment it should be noted that it is a kind of decomposition of the 8 bit grey level in 8 or more lighting sub-periods. An optimisation of such a picture encoding will have, indeed, a positive effect on the false contour effect. Nevertheless, such a solution can only slightly reduce the false contour effect amplitude but in any case the effect will still occur and will be perceivable. Furthermore, the sub-field organization is not a simple matter of design choice. The more sub-fields are allowed the less luminance the panel will be able to produce. So optimisation of the sub-field organization is only possible in a narrow range and will not eliminate this effect alone.
A second approach for the solution of above mentioned problem is known under the expression “pulse equalization technique”. This technique is a more complex one. It uses equalizing pulses, which are added or separated from the TV signal when disturbances of grey scale are foreseen. In addition, since the fact that the false contour effect is motion relevant, different pulses for each possible speed are necessary. That leads to the need of a big memory storing a large number of look-up tables (LUT) for each speed and there is a need of a motion estimator. Furthermore, since the false contour effect depends on the sub-field organization, the pulses have to be recalculated for each new sub-field organization. However, the big disadvantage of this technique results from the fact that the equalizing pulses add failures to the picture to compensate for a failure appearing on the eye retina. Additionally, when the motion increases in the picture, there is a need to add more pulses to the picture and that leads to conflicts with picture contents in case of very fast motion.
A further approach which is described in prior art documents, like EP-A-0 980 059, is based on a detection of the movements in the picture (displacement of the eye focus area) and the spreading of the right sub-field lighting periods over this displacement in order to ensure that the eye will only perceive the correct information through its movement. This solution requires a motion estimator, which delivers motion vector data for the pixels or pixel blocks. For each pixel the corresponding motion vector data is used to shift the entries in the sub-field code word in the direction of the motion vector. Thus the sub-field code words are corrected or recoded. The solution is good, and gives a good picture quality but, of course, has a need of an implementation of a motion estimator, which makes the high speed motion estimation. This motion estimator is relatively costly and not easy to implement.
Another approach for compensating the dynamic false contour effect is based on a new type of sub-field coding. Which is called “incremental sub-field coding”. The incremental sub-field coding method is disclosed for example in the European Patent Publication EP-A-0 952 569. In this type of sub-field coding method, there are only some basic sub-field code words used for the grey scale portrayal rendition. This means, that in the case of 8 bit video data there are not 256 different sub-field code words for the possible video levels, but instead only a few sub-field code words with specific characteristic for some distinct video levels and the remaining video levels are rendered by some optimised dithering or error diffusion technique. The speciality of the incremental code is that in each case there is never one sub-field inactivated between two consecutive activated sub-fields and never one sub-field activated between two consecutive inactivated sub-fields. With this characteristic the incremental code has the advantage, that false contour effect is no longer a problem due to the fact that sub-field code words for similar video levels cannot deviate at various bit positions.
The structure of such sub-field code words is very specific and varies from code word to code word in only one sub-field entry. This means that when there is a smooth transition of video levels like in a homogenous surface, as skin, then there will no longer occur the changes in the structure of sub-field code words, which can cause false contour effect. The number of available video levels is, however, substantively reduced so that a poor grey scale rendition results. To improve this grey scale rendition, a dithering technique is required, which brings back some of the lost video levels. It is hardly possible to bring back all the lost video levels with such a dithering technique or error diffusion technique in the case of this specific sub-field coding, where the number of grey levels is reduced to the number of sub-fields in the sub-field organization.